Alkylene glycols, in particular monoalkylene glycols, are of established commercial interest. For example, monoalkylene glycols are used in anti-freeze compositions, as solvents and as base materials in the production of polyalkylene terephthalates e.g. for fibers or bottles.
The production of alkylene glycols by liquid phase hydrolysis of alkylene oxide is known. The hydrolysis is generally performed by adding a large excess of water, e.g. 20 to 25 moles of water per mole of alkylene oxide. The reaction is considered to be a nucleophilic substitution reaction, whereby opening of the alkylene oxide ring occurs, water acting as the nucleophile. Because the primarily formed monoalkylene glycol also acts as a nucleophile, as a rule a mixture of monoalkylene glycol, dialkylene glycol and higher alkylene glycols is formed. In order to increase the selectivity to monoalkylene glycol, it is necessary to suppress the secondary reaction between the primary product and the alkylene oxide, which competes with the hydrolysis of the alkylene oxide.
One effective means for suppressing the secondary reaction is to increase the relative amount of water present in the reaction mixture. Although this measure improves the selectivity towards the production of the monoalkylene glycol, it creates a problem in that large amounts of water have to be removed for recovering the product.
Considerable efforts have been made to find an alternative means for increasing the reaction selectivity without having to use a large excess of water. The hydrolysis of alkylene oxides to alkylene glycols can be performed with a smaller excess of water in a catalytic system. Therefore, these efforts have usually focused on the selection of more active hydrolysis catalysts and various catalysts have been disclosed in the literature.
Both acid and alkaline hydrolysis catalysts have been investigated, whereby it would appear that the use of acid catalysts enhances the reaction rate without significantly affecting the selectivity, whereas by using alkaline catalysts generally lower selectivities with respect to the monoalkylene glycol are obtained.
Catalytic processes, promoting a higher selectivity to monoalkylene glycol product at reduced water levels are known (e.g. EP-A-0,156,449, U.S. Pat. No. 4,982,021, U.S. Pat. No. 5,488,184, U.S. Pat. No. 6,153,801 and U.S. Pat. No. 6,124,508). Such catalysts generally comprise a strongly basic (anionic) exchange resin, often with quaternary ammonium or quaternary phosphonium electropositive complexing sites, coordinated with one or more anions (e.g. metalate, halogen, bicarbonate, bisulfite or carboxylate), and are generally most effective when used at elevated temperature.
A drawback shared by anionic exchange resins is, however, their limited tolerance to heat and their susceptibility to swelling. In International Patent Specification No. WO 02/098828 this swelling is attributed to thermal degradation of the catalyst resin and reaction of the resin with EO.
Catalyst swelling is problematic as it can result in the flow of reactants through the reactor being slowed or blocked. Therefore, efforts have been made to develop methods for reducing thermal swelling.
In the prior art, there are disclosed methods for reducing thermal swelling of hydrolysis catalysts.
In U.S. Pat. No. 6,137,015 it is described how processes comprising anion exchange resins, e.g. as described in U.S. Pat. No. 5,488,184, suffer from undesirable swelling, particularly at temperatures greater than 95° C. This document further describes a method of minimizing such swelling comprising adding to the reaction mixture a combination of additives comprising carbon dioxide and a base in an amount sufficient to maintain a pH between 5.0 and 9.0.
In EP-A-1,140,749, it is disclosed that enhancing the stability of basic ion exchange resins by adding a relatively small amount of an acidic ion exchange resin reduces catalyst swelling.
U.S. Pat. No. 6,160,187 proposes a method of minimizing the swelling of an anion exchange resin by using an adiabatic reactor.
US-A-2002/082456, describes a process for reducing the rate of swelling of an anion exchange resin-based catalyst by recycling reactor output from a reactor containing a catalyst based on an anion exchange resin back through the same reactor.
The efforts in the literature have thus concentrated on means to reduce thermal swelling which has hitherto been taught to be the major cause of catalyst swelling. However, despite the above proposals, the problem of reduction of catalyst swelling has not been adequately solved, and catalyst swelling is still a significant hindrance to the provision of a successful commercial-scale catalytic hydrolysis process, utilizing ion exchange resin catalysts.